[7.9] Endpoint ECS mapping

[caitlinbetz]

Description

Document ECS mapping for Endpoint
Similar to: https://www.elastic.co/guide/en/beats/auditbeat/master/exported-fields-ecs.html
We will link to this in our package definition

Acceptance Test Criteria

List all the ATC of each action and its intended result.
As a user, when [action (e.g., viewing, clicking, selecting, etc.)] the [insert the expected result].
If the doc issue includes a procedure, number the steps in sequential order.

Notes

• Add the "Team:Docs" label to new issues.
• Be sure to add any necessary screenshots for clarity.
• Include any conditions or caveats that may affect customers.